Liquid ring vacuum pumps, as exemplified by Roe et al U.S. Reissue Pat. No. 29,747, which is incorporated herein by reference, use "seal water" for two purposes, first to form a liquid ring of working pistons that compress the gas and push it out of the pump, and second to form a seal between high pressure gas being discharged and low pressure gas entering the pump. This seal is formed in the angular segment land area of the 360 degree cycle, where the liquid ring pistons contact the cone surface. As used herein, the land, or land area, of the cone shall mean that portion of the cone which is in closest communication with the working water pistons. The efficiency of the pump depends on the seal created by both the clearance of the metal surfaces of the rotor vanes and cone surface and the pistons contacting the cone land area.
Shaft packing rings in a "stuffing box" require water for both sealing and cooling. If secondary (or recycled) water is pumped into the liquid ring pump through the center of the cone, then fresh water is piped separately into the stuffing box to avoid erosion of the shaft.
Metal parts of a liquid ring pump, particularly the vane surface at the inner tapered diameter of vanes and the cone land area surface, become worn during operation, causing an opening of clearances and subsequent loss of efficiency. After an extended time in operation, costly replacement or repair of the worn parts is required to rebuild the pump in order for it to perform anywhere approaching its original efficiency. Using only clean fresh water with known liquid ring vacuum pumps will reduce the cause for repair of such pumps, but with ever increasing costs for use of fresh water, and sometimes limited fresh water availability, use of 100% fresh water has become expensive or prohibitive. Also, the pre-treatment/filtration equipment necessary to remove suspended particulates from the secondary plant water to produce water with the degree of cleanliness that would minimize the erosive wear is both costly to purchase and expensive to operate and maintain. Additionally, a problem with using secondary plant water for sealing water is that it contains erosive particulates and will wear away the metal, both on the inner surface of the rotor vanes at the small diameter end of the tapered cone, and on the land area of the cone. This loss of metal weakens the liquid seal in the land area and causes an early loss of pump efficiency, thereby causing the need for costly pump repairs.
It is not recommended to use secondary plant water to seal and cool the shaft packing rings, since secondary plant water contains particles that would be captured between the shaft and packing, and cause excessive wear of the shaft material. Clean fresh water is piped separately to the stuffing box that holds the packing rings.
Secondary plant water can be used to form the piston of the pump, thereby saving fresh water. Secondary plant water contains sediment and particulates which over time will accumulate in the manifold delivery system. The result is an inadequate supply of secondary water to assure adequate cooling. To correct this condition it would be necessary to remove the manifold to clean out the accumulated sediment which is costly both in terms of down time of the pump and in labor costs.